Golf ball

ABSTRACT

The present invention provides a golf ball which includes (I) a first member composed of a material molded under heat from a rubber composition of (a) a base rubber composed primarily of a mixture of at least 60 wt % of a polybutadiene having a stress relaxation time (T 80 ), as defined below, of less than 4 seconds and up to 40 wt % of a polybutadiene having a stress relaxation time (T 80 ) of at least 4 seconds, (b) an unsaturated carboxylic acid and/or a metal salt thereof, and (c) an organic peroxide; and (II) a second member composed of a specific, highly-neutralized, resin composition. The golf ball of the invention, by being thus constituted, has an excellent resilience for the ball as a whole, a good, soft, feel on impact, and imparts an excellent spin performance, thereby enabling an increased distance to be achieved. In addition, it has a high manufacturability.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of application Ser. No. 11/336,790 filed on Jan. 23, 2006, now U.S. Pat. No. 7,534,835 issued on May 19, 2009, the entire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a golf ball which has an excellent rebound and good manufacturability.

Efforts to impart golf balls with an excellent rebound have until now focused on one or more indicator of the polybutadiene used as the base rubber, such as the Mooney viscosity, polymerization catalyst, solvent viscosity and molecular weight distribution, and attempted to optimize these indicators. See, for example, Patent Document 1: JP-A 2004-292667; Patent Document 2: U.S. Pat. No. 6,818,705; Patent Document 3: JP-A 2002-355336; Patent Document 4: JP-A 2002-355337; Patent Document 5: JP-A 2002-355338; Patent Document 6: JP-A 2002-355339; Patent Document 7: JP-A 2002-355340; and Patent Document 8: JP-A 2002-356581.

For example, Patent Document 1 (JP-A 2004-292667) describes, as a base rubber for golf balls, a polybutadiene having a Mooney viscosity of 30 to 42 and a molecular weight distribution (Mw/Mn) of 2.5 to 3.8. Patent Document 2 (U.S. Pat. No. 6,818,705) describes, for the same purpose, a polybutadiene having a molecular weight of at least 200,000 and a resilience index of at least 40.

However, many golfers desire golf balls capable of attaining a longer travel distance, and so a need exists for the development of golf balls having an even better rebound.

Obtaining a golf ball having a high rebound has required until now the use of a high Mooney viscosity rubber, which has a poor workability and inevitably lowers the manufacturability of the ball. A desire has thus existed for the development of a high-rebound golf ball without reducing the ease of manufacture.

SUMMARY OF THE INVENTION

The present invention thus relates to a golf ball which, along with having an excellent rebound and being imparted with a good, soft feel on impact and an excellent spin performance that enables an increased distance to be achieved, also has a high manufacturability.

As a result of extensive investigations, the inventors have discovered that, in the production of golf balls wherein a hot-molded rubber composition containing a base rubber, an unsaturated carboxylic acid and/or a metal salt thereof, and an organic peroxide serves as a first member (I), by including in the base rubber specific amounts of two kinds of polybutadiene having specific T₈₀ values and by having a specific, highly neutralized resin composition serve as a second member (II), it is possible to retain a good ball rebound and to impart a good, soft feel on impact and an excellent spin performance that enables an increased distance to be achieved, in addition to which the ball has a high manufacturability.

Accordingly, the invention provides the following golf ball.

[1] A golf ball comprising a plurality of members, which members include

(I) a first member comprised of a material molded under heat from a rubber composition comprising (a) a base rubber composed primarily of a mixture of at least 60 wt % of a polybutadiene having a stress relaxation time (T₈₀), defined as the length of time it takes from the moment rotor rotation is stopped immediately after measurement of the ML₁₊₄ (100° C.) value (the Mooney viscosity measured at 100° C. in accordance with ASTM D-1646-96) for the ML₁₊₄ value to decrease 80%, of less than 4 seconds and up to 40 wt % of a polybutadiene having a stress relaxation time (T₈₀) of at least 4 seconds, (b) an unsaturated carboxylic acid and/or a metal salt thereof, and (c) an organic peroxide; and

(II) a second member formed primarily of a resin composition obtained by mixing:

-   -   100 parts by weight of a base resin composed primarily of (A-I)         an olefin-unsaturated carboxylic acid-unsaturated carboxylic         acid ester random terpolymer and/or a metal salt thereof, and/or         (A-II) an olefin-unsaturated carboxylic acid random copolymer         and/or a metal salt thereof,     -   (B) from 5 to 170 parts by weight of a fatty acid or fatty acid         ester having a molecular weight of from 280 to 1500, and     -   (C) from 0.1 to 10 parts by weight of a basic inorganic metal         compound capable of neutralizing acid groups in components A and         B.         [2] The golf ball of [1], wherein the rubber composition further         comprises (d) an organosulfur compound.         [3] The golf ball of [1], wherein the polybutadiene having a         stress relaxation time (T₈₀) of less than 4 seconds and the         polybutadiene having a stress relaxation time (T₈₀) of at least         4 seconds are both polybutadienes prepared using a rare-earth         catalyst.         [4] The golf ball of [1], wherein at least the polybutadiene         having a stress relaxation time (T₈₀) of less than 4 seconds or         the polybutadiene having a stress relaxation time (T₈₀) of at         least 4 seconds is a polybutadiene prepared using a rare-earth         catalyst.         [5] The golf ball of [4], wherein at least one of the         polybutadienes is a polybutadiene prepared by polymerization         using a rare-earth catalyst, followed by terminal modification.         [6] The golf ball of [1], wherein the olefin-unsaturated         carboxylic acid-unsaturated carboxylic acid ester random         terpolymer and/or a metal salt thereof serving as component A-I         has a weight-average molecular weight (Mw) of at least 100,000         but not more than 200,000, and a weight-average molecular weight         (Mw) to number-average molecular weight (Mn) ratio of at least 3         but not more than 7.0.         [7] The golf ball of [1], wherein the olefin-unsaturated         carboxylic acid random copolymer and/or a metal salt thereof         serving as component A-II has a weight-average molecular weight         (Mw) of at least 150,000 but not more than 200,000, and a         weight-average molecular weight (Mw) to number-average molecular         weight (Mn) ratio of at least 3 but not more than 7.0.         [8] The golf ball of [1], wherein the resin composition has a         melt flow rate, as measured in accordance with JIS-K6760 (test         temperature, 190° C.; test load, 21 N (2.16 kgf)), of at least 1         g/10 min but not more than 30 g/10 min.         [9] The golf ball of [1], wherein the terpolymer of component         A-I accounts for a proportion of the overall base resin of from         100 to 30 wt %, and the copolymer of component A-II accounts for         a proportion of the overall base resin of from 0 to 70 wt %.         [10] The golf ball of [1] which is a golf ball having a core, an         inner cover layer and an outer cover layer, wherein the first         member (I) is used as the core and the second member (II) is         used as the inner cover layer.         [11] The golf ball of [10], wherein the inner cover layer has a         Shore D hardness of from 50 to 80, and the outer cover layer has         a Shore D hardness which is from 35 to 60 and is lower than the         Shore D hardness of the inner cover layer.

DETAILED DESCRIPTION OF THE INVENTION

The invention is described more fully below. The golf ball of the invention includes, as a first member (referred to below as “first member (I)”), a material molded under heat from a rubber composition of (a) a base rubber composed primarily of a mixture of at least 60 wt % of a polybutadiene having a stress relaxation time (T₈₀), as defined below, of less than 4 seconds (which polybutadiene is sometimes abbreviated below as “BR1”) with up to 40 wt % of a polybutadiene having a stress relaxation time (T₈₀) of at least 4 seconds (sometimes abbreviated below as “BR2”), (b) an unsaturated carboxylic acid and/or a metal salt thereof, and (c) an organic peroxide; and includes also a second member (referred to below as “first member (II)”) formed primarily of a resin composition obtained by mixing:

100 parts by weight of a base resin composed primarily of (A-I) an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymer and/or a metal salt thereof, and/or (A-II) an olefin-unsaturated carboxylic acid random copolymer and/or a metal salt thereof,

(B) from 5 to 170 parts by weight of a fatty acid or fatty acid ester having a molecular weight of from 280 to 1500, and

(C) from 0.1 to 10 parts by weight of a basic inorganic metal compound capable of neutralizing acid groups in components A and B.

The stress relaxation time (T₈₀), which is expressed in seconds, is the length of time it takes from the moment rotor rotation is stopped immediately after measurement of the ML₁₊₄ (100° C.) value (the Mooney viscosity measured at 100° C. in accordance with ASTM D1646-96) for the ML₁₊₄ value to decrease 80%.

This indicator is described in section 13.1.3.1 of ASTM D1646-96.

At first, the above first ball member (I) is explained in detail.

The term “Mooney viscosity” used herein refers to an industrial indicator of viscosity as measured with a Mooney viscometer, which is a type of rotary plastometer. The unit symbol used is ML₁₊₄ (100° C.), where “M” stands for Mooney viscosity, “L” stands for large rotor (L-type), “1+4” stands for a pre-heating time of 1 minute and a rotor rotation time of 4 minutes, and “100° C.” indicates that measurement was carried out at a temperature of 100° C.

In the practice of the invention, the polybutadiene in component (a) includes a polybutadiene BR1 having a stress relaxation time (T₈₀) of less than 4 seconds. The T₈₀ value is preferably 3.5 seconds or less, and more preferably 3 seconds or less, but preferably at least 1 second, and more preferably at least 1.5 seconds.

The polybutadiene in component (a) also includes a polybutadiene BR2 having a stress relaxation time (T₈₀) of at least 4 seconds. The T₈₀ value is preferably at least 4.5 seconds, and more preferably at least 5 seconds, but preferably not more than 20 seconds, and more preferably not more than 10 seconds.

BR1 has a Mooney viscosity (ML₁₊₄ (100° C.)) which is generally at least 20, preferably at least 30, more preferably at least 40, and most preferably at least 50, but generally not more than 80, preferably not more than 70, more preferably not more than 65, and most preferably not more than 60. If the Mooney viscosity is too large, the workability may worsen. On the other hand, if the Mooney viscosity is too small, the resilience may decrease.

BR2 has a Mooney viscosity (ML₁₊₄ (100° C.)) which is generally at least 20, preferably at least 30, and more preferably at least 40, but generally not more than 70, and preferably not more than 60. If the Mooney viscosity is too large, the workability may worsen. On the other hand, if the Mooney viscosity is too small, the resilience may decrease.

BR1 and BR2 have cis-1,4 bond contents of generally at least 60%, preferably at least 80%, more preferably at least 90%, and most preferably at least 95%, and have 1,2-vinyl bond contents of generally at most 3%, preferably at most 2%, more preferably at most 1.5%, and most preferably at most 1.3%. At cis-1,4 bond contents or 1,2-vinyl bond contents outside of these ranges, the resilience may decrease.

The polybutadiene mixture in the invention is composed of the above polybutadienes BR1 and BR2. Using such a mixture enables efficient production to be carried out while maintaining a good resilience, thus imparting the inventive golf balls with an outstanding rebound.

The proportion of BR2 in the polybutadiene mixture is 40 wt % or less, preferably 35 wt % or less, more preferably 30 wt % or less, and even more preferably 25 wt % or less. Too large a proportion lowers the rebound.

In the practice of the invention, the base rubber is composed primarily of the above-described polybutadiene mixture. That is, the polybutadiene mixture accounts for at least 50 wt %, preferably at least 80 wt %, more preferably at least 90 wt %, and even up to 100 wt %, of the base rubber. If this proportion is too low, the resilience may decrease.

No particular limitation is imposed on rubber compounds other than the above polybutadienes BR1 and BR2 which may be included in the base rubber. Illustrative examples of such other rubber compounds include styrene-butadiene rubber (SBR), natural rubber, polyisoprene rubber, and ethylene-propylene-diene rubber (EPDM)). These may be used individually or as combinations of two or more.

The Mooney viscosity of such additional rubber compounds is generally not more than 55, preferably not more than 50, more preferably not more than 47, and even more preferably not more than 45, but generally at least 10, preferably at least 20, more preferably at least 25, and most preferably at least 30. At a Mooney viscosity outside of the above range, such additional rubber compounds may compromise the extrusion workability of the rubber composition and lower the resilience of the hot-molded material.

To achieve a high resilience, it is preferable for both BR1 and BR2 in the invention to be polybutadienes synthesized using a rare-earth catalyst.

A known rare-earth catalyst may be used for this purpose. Exemplary rare-earth catalysts include those made up of a combination of a lanthanide series rare-earth compound, an organoaluminum compound, an alumoxane, a halogen-bearing compound, and an optional Lewis base.

Examples of suitable lanthanide series rare-earth compounds include halides, carboxylates, alcoholates, thioalcoholates and amides of atomic number 57 to 71 metals.

Organoaluminum compounds that may be used include those of the formula AlR¹R²R³ (wherein R¹, R² and R³ are each independently a hydrogen or a hydrocarbon group of 1 to 8 carbons).

Preferred alumoxanes include compounds of the structures shown in formulas (I) and (II) below. The alumoxane association complexes described in Fine Chemical 23, No. 9, 5 (1994), J. Am. Chem. Soc. 115, 4971 (1993), and J. Am. Chem. Soc. 117, 6465 (1995) are also acceptable.

(In the above formulas, R⁴ is a hydrocarbon group having 1 to 20 carbon atoms, and n is 2 or a larger integer).

Examples of halogen-bearing compounds that may be used include aluminum halides of the formula AlX_(n)R_(3-n) (wherein X is a halogen; R is a hydrocarbon group of 1 to 20 carbons, such as an alkyl, aryl or aralkyl; and n is 1, 1.5, 2 or 3); strontium halides such as Me₃SrCl, Me₂SrCl₂, MeSrHCl₂ and MeSrCl₃; and other metal halides such as silicon tetrachloride, tin tetrachloride and titanium tetrachloride.

The Lewis base can be used to form a complex with the lanthanide series rare-earth compound. Illustrative examples include acetylacetone and ketone alcohols.

In the practice of the invention, the use of a neodymium catalyst in which a neodymium compound serves as the lanthanide series rare-earth compound is particularly advantageous because it enables a polybutadiene rubber having a high cis-1,4 bond content and a low 1,2-vinyl bond content to be obtained at an excellent polymerization activity. Preferred examples of such rare-earth catalysts include those mentioned in JP-A 11-35633.

The polymerization of butadiene in the presence of a rare-earth catalyst may be carried out by bulk polymerization or vapor phase polymerization, either with or without the use of solvent, and at a polymerization temperature in a range of generally from −30 to +150° C., and preferably from 10 to 100° C.

To manufacture golf balls of a stable quality, it is desirable for at least one of the above polybutadienes BR1 and BR2 in the invention to be a terminal-modified polybutadiene obtained by polymerization using the above-described rare-earth catalyst, followed by the reaction of a terminal modifier with active end groups on the polymer.

A known terminal modifier may be used for this purpose. Illustrative examples include compounds of types (1) to (6) below.

(1) Halogenated organometallic compounds, halogenated metallic compounds and organometallic compounds of the general formulas R⁵ _(n)M′X_(4-n), M′X₄, M′X₃, R⁵ _(n)M′(—R⁶—CoOR⁷)_(4-n) or R⁵ _(n)M′(—R⁶—CoR⁷)_(4-n) (wherein R⁵ and R⁶ are each independently a hydrocarbon group of 1 to 20 carbons; R⁷ is a hydrocarbon group of 1 to 20 carbons which may contain pendant carbonyl or ester groups; M′ is a tin, silicon, germanium or phosphorus atom; X is a halogen atom; and n is an integer from 0 to 3); (2) heterocumulene compounds having on the molecule a Y═C═Z linkage (wherein Y is a carbon, oxygen, nitrogen or sulfur atom; and Z is an oxygen, nitrogen or sulfur atom); (3) three-membered heterocyclic compounds containing on the molecule the following bonds

(wherein Y is an oxygen, nitrogen or sulfur atom); (4) halogenated isocyano compounds; (5) carboxylic acids, acid halides, ester compounds, carbonate compounds and acid anhydrides of the formula R⁸—(COOH)_(m), R⁹(COX)_(m), R¹⁰—(COO—R¹¹), R¹²—OCOO—R¹³, R¹⁴—(COOCO—R¹⁵)_(m) or

(wherein R⁸ to R¹⁶ are each independently a hydrocarbon group of 1 to 50 carbons, X is a halogen atom, and m is an integer from 1 to 5); and (6) carboxylic acid metal salts of the formula R¹⁷ ₁ M″(OCOR¹⁸)₄₋₁, R¹⁹ ₁M″(OCO—R²⁰—COOR²¹)₄₋₁ or

(wherein R¹⁷ to R²³ are each independently a hydrocarbon group of 1 to 20 carbons, M″ is a tin, silicon or germanium atom, and the letter 1 is an integer from 0 to 3).

Specific examples of the above terminal modifiers (1) to (6) and methods for their reaction are described in, for example, JP-A 11-35633 and JP-A 7-268132.

In the practice of the invention, the polybutadiene BR2 may be synthesized with a group VIII catalyst. Exemplary group VIII catalysts include the following nickel catalysts and cobalt catalysts.

Examples of suitable nickel catalysts include single-component systems such as nickel-kieselguhr, binary systems such as Raney nickel/titanium tetrachloride, and ternary systems such as nickel compound/organometallic compound/boron trifluoride etherate. Exemplary nickel compounds include reduced nickel on a carrier, Raney nickel, nickel oxide, nickel carboxylate and organonickel complex salts. Exemplary organometallic compounds include trialkylaluminum compounds such as triethylaluminum, tri-n-propylaluminum, triisobutylaluminum and tri-n-hexylaluminum; alkyllithium compounds such as n-butyllithium, sec-butyllithium, tert-butyllithium and 1,4-dilithiumbutane; and dialkylzinc compounds such as diethylzinc and dibutylzinc.

Examples of suitable cobalt catalysts include cobalt and cobalt compounds such as Raney cobalt, cobalt chloride, cobalt bromide, cobalt iodide, cobalt oxide, cobalt sulfate, cobalt carbonate, cobalt phosphate, cobalt phthalate, cobalt carbonyl, cobalt acetylacetonate, cobalt diethyldithiocarbamate, cobalt anilinium nitrite and cobalt dinitrosyl chloride. It is particularly advantageous to use these compounds in combination with, for example, a dialkylaluminum monochloride such as diethylaluminum monochloride or diisobutylaluminum monochloride; a trialkylaluminum such as triethylaluminum, tri-n-propylaluminum, triisobutylaluminum or tri-n-hexylaluminum; an alkylaluminum sesquichloride such as ethylaluminum sesquichloride; or aluminum chloride.

Polymerization using the above group VIII catalysts, and particularly a nickel or cobalt catalyst, can be carried out by a process in which the catalyst typically is continuously charged into a reactor together with a solvent and butadiene monomer, and the reaction conditions are suitably selected, such as a reaction temperature in a range of 5 to 60° C. and a reaction pressure in a range of atmospheric pressure to 70 plus atmospheres, so as to yield a product having the above-indicated Mooney viscosity.

Above component (b) may be an unsaturated carboxylic acid, specific examples of which include acrylic acid, methacrylic acid, maleic acid and fumaric acid. Acrylic acid and methacrylic acid are especially preferred. Alternatively, it may be the metal salt of an unsaturated carboxylic acid, examples of which include the zinc and magnesium salts of unsaturated fatty acids, such as zinc methacrylate and zinc acrylate. The use of zinc acrylate is especially preferred.

The content of above component (b) per 100 parts by weight of the base rubber is generally at least 10 parts by weight, and preferably at least 15 parts by weight, but generally not more than 60 parts by weight, preferably not more than 50 parts by weight, even more preferably not more than 45 parts by weight, and most preferably not more than 40 parts by weight. Too much component (b) will make the material molded under heat from the rubber composition too hard, giving the golf ball an unpleasant feel on impact. On the other hand, too little will result in a lower rebound.

Above component (c) may be a commercially available product, suitable examples of which include Percumyl D (produced by NOF Corporation), Perhexa 3 C(NOF Corporation), and Luperco 231XL (Atochem Co.). If necessary, a combination of two or more different organic peroxides may be used.

The amount of component (c) per 100 parts by weight of the base rubber is generally at least 0.1 part by weight, and preferably at least 0.3 part by weight, but generally not more than 5 parts by weight, preferably not more than 4 parts by weight, more preferably not more than 3 parts by weight, and most preferably not more than 2 parts by weight. Too much or too little component (c) may make it impossible to obtain a suitable hardness distribution, resulting in a poor feel on impact, poor durability and a poor rebound.

To further improve the resilience, it is desirable for the rubber composition in the invention to include also the following component (d):

(d) an organosulfur compound.

Examples of such organosulfur compounds include thiophenols, thionaphthols, halogenated thiophenols, and metal salts thereof. Specific examples include the zinc salts of pentachlorothiophenol, pentafluorothiophenol, pentabromothiophenol, p-chlorothiophenol and pentachlorothiophenol; and diphenylpolysulfides, dibenzylpolysulfides, dibenzoylpolysulfides, dibenzothiazoylpolysulfides and dithiobenzoylpolysulfides having 2 to 4 sulfurs. These may be used singly or as combinations of two or more thereof. Diphenyldisulfide and the zinc salt of pentachlorothiophenol, used either alone or together, are especially preferred.

The amount of component (d) included per 100 parts by weight of the base rubber is generally at least 0.1 part by weight, preferably at least 0.2 part by weight, and more preferably at least 0.5 part by weight, but not more than 5 parts by weight, preferably not more than 4 parts by weight, and more preferably not more than 3 parts by weight. Too much organosulfur compound may make the material molded under heat from the rubber composition too soft, whereas too little may make an improved resilience difficult to achieve.

The rubber composition used in the invention may additionally include such additives as inert fillers and antioxidants. Illustrative examples of suitable inert fillers include zinc oxide, barium sulfate and calcium carbonate. The amount of inert filler included per 100 parts by weight of the base rubber is generally at least 5 parts by weight, preferably at least 7 parts by weight, more preferably at least 10 parts by weight, and most preferably at least 13 parts by weight, but generally not more than 80 parts by weight, preferably not more than 50 parts by weight, more preferably not more than 45 parts by weight, and most preferably not more than 40 parts by weight. Too much or too little inorganic filler may make it impossible to obtain a proper golf ball weight and a suitable rebound.

To increase the rebound, it is desirable for the inorganic filler to include zinc oxide in an amount of at least 50 wt %, preferably at least 75 wt %, and most preferably 100 wt % (the zinc oxide being 100% of the inorganic filler).

The zinc oxide has an average particle size (by air permeametry) of preferably at least 0.01 μm, more preferably at least 0.05 μm, and most preferably at least 0.1 μm, but preferably not more than 2 μm, and more preferably not more than 1 μm.

Examples of suitable antioxidants include 2,2′-methylenebis(4-methyl-6-t-butylphenol) (available from Ouchi Shinko Chemical Industry Co., Ltd. as the commercial product Nocrac NS-6) and 2,2′-methylenebis(4-ethyl-6-t-butylphenol) (available from Ouchi Shinko Chemical Industry Co., Ltd. as the commercial product Nocrac NS-5). To achieve a good rebound and durability, it is recommended that the amount of antioxidant included per 100 parts by weight of the base rubber be more than 0 part by weight, preferably at least 0.05 part by weight, more preferably at least 0.1 part by weight, and most preferably at least 0.2 part by weight, but not more than 3 parts by weight, preferably not more than 2 parts by weight, more preferably not more than 1 part by weight, and most preferably not more than 0.5 part by weight.

The material molded under heat from the rubber composition in the present invention can be obtained by vulcanizing and curing the rubber composition using a method of the same sort as that used on prior-art rubber compositions for golf balls. Vulcanization may be carried, for example, at a temperature of from 100 to 200° C. for a period of 10 to 40 minutes.

The material molded under heat from the rubber composition in the invention has a hardness difference, obtained by subtracting the JIS-C hardness at the center of the hot-molded material from the JIS-C hardness at the surface of the molded material, of generally at least 15, preferably at least 16, more preferably at least 17, and even more preferably at least 18, but generally not more than 50, and preferably not more than 40. Setting the hardness within this range is desirable for achieving a golf ball having a soft feel and a good rebound and durability.

It is recommended that the hot-molded material obtained from the rubber composition in the invention, regardless of which of the subsequently described golf balls in which it is used, have a deflection when subjected to loading from an initial load of 98 N (10 kgf) to a final load of 1275 N (130 kgf), of generally at least 2.0 mm, preferably at least 2.5 mm, and more preferably at least 2.8 mm, but not more than 6.0 mm, preferably not more than 5.5 mm, even more preferably not more than 5.0 mm, and most preferably not more than 4.5 mm. Too small a deflection may worsen the feel of the ball on impact and, particularly on long shots such as with a driver in which the ball incurs a large deformation, may subject the ball to an excessive rise in spin, shortening the distance of travel. On the other hand, a hot-molded material that is too soft deadens the feel of the golf ball when played, compromises the rebound of the ball, resulting in a shorter distance, and gives the ball a poor durability to cracking with repeated impact.

In the practice of the invention, when the hot-molded material serves as a solid core, it is recommended that the solid core have a diameter of at least 30.0 mm, preferably at least 32.0 mm, more preferably at least 35.0 mm, and most preferably at least 37.0 mm, but not more than 41.0 mm, preferably not more than 40.5 mm, more preferably not more than 40.0 mm, and most preferably not more than 39.5 mm.

In particular, the solid core in a solid two-piece golf ball has a diameter of generally at least 37.0 mm, preferably at least 37.5 mm, more preferably at least 38.0 mm, and most preferably at least 38.5 mm, but generally not more than 41.0 mm, preferably not more than 40.5 mm, and more preferably not more than 40.0 mm.

Similarly, it is recommended that such a solid core in a solid three-piece golf ball have a diameter of at least 30.0 mm, preferably at least 32.0 mm, more preferably at least 34.0 mm, and most preferably at least 35.0 mm, but not more than 40.0 mm, preferably not more than 39.5 mm, and even more preferably not more than 39.0 mm.

It is recommended that the solid core have a specific gravity of generally at least 0.9, preferably at least 1.0, and more preferably at least 1.1, but generally not more than 1.4, preferably not more than 1.3, and more preferably not more than 1.2.

Next, the golf ball of the present invention includes the above-described molded and vulcanized rubber composition as the first member (I) thereof, and includes also the second member (II) described below.

The second member (II) is formed primarily of a resin composition obtained by mixing:

100 parts by weight of a base resin composed primarily of (A-I) an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymer and/or a metal salt thereof, and/or (A-II) an olefin-unsaturated carboxylic acid random copolymer and/or a metal salt thereof,

(B) from 5 to 170 parts by weight of a fatty acid or fatty acid ester having a molecular weight of from 280 to 1500, and

(C) from 0.1 to 10 parts by weight of a basic inorganic metal compound capable of neutralizing acid groups in components A and B.

The resin mixture is described below.

The olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymer and/or a metal salt thereof serving as component A-I has a weight-average molecular weight (Mw) of preferably at least 100,000, more preferably at least 110,000, and even more preferably at least 120,000, but preferably not more than 200,000, more preferably not more than 190,000, and even more preferably not more than 170,000. The weight-average molecular weight (Mw) to number-average molecular weight (Mn) ratio of the above terpolymer is preferably at least 3, and more preferably at least 4, but preferably not more than 7.0, and more preferably not more than 6.5.

Here, the weight-average molecular weight (Mw) and number-average molecular weight (Mn) are values calculated relative to polystyrene in gel permeation chromatography (GPC). A word of explanation is needed here concerning GPC molecular weight measurement. It is not possible to directly take GPC measurements for binary copolymers and terpolymers because the molecules thereof are adsorbed to the GPC column owing to the unsaturated carboxylic acid groups within the molecules. Instead, the unsaturated carboxylic acid groups are generally converted to esters, following which GPC measurement is carried out and the polystyrene-equivalent average molecular weights Mw and Mn are calculated.

Component A-I and component A-II are olefin-containing copolymers. The olefins in these copolymers are exemplified by olefins in which the number of carbons is at least 2, but not more than 8, and preferably not more than 6. Illustrative examples of such olefins include ethylene, propylene, butene, pentene, hexene, heptene and octene. Ethylene is especially preferred.

Illustrative examples of the unsaturated carboxylic acids in component A-I and component A-II include acrylic acid, methacrylic acid, maleic acid and fumaric acid. Acrylic acid and methacrylic acid are especially preferred.

The unsaturated carboxylic acid ester in component A-I may be, for example, a lower alkyl ester of any of the above-mentioned unsaturated carboxylic acids. Illustrated examples include methyl methacrylate, ethyl methacrylate, propyl methacrylate, butyl methacrylate, methyl acrylate, ethyl acrylate, propyl acrylate and butyl acrylate. Butyl acrylate (n-butyl acrylate, i-butyl acrylate) is especially preferred.

The random terpolymer serving as component A-I may be obtained by random copolymerization of the above ingredients in accordance with a known method. Here, it is recommended that the content of unsaturated carboxylic acid (acid content) included in the random terpolymer be generally at least 2 wt %, preferably at least 6 wt %, and more preferably at least 8 wt %, but not more than 25 wt %, preferably not more than 20 wt %, and even more preferably not more than 15 wt %. At a low acid content, the rebound may decrease, whereas at a high acid content, the processability of the material may decrease.

The terpolymer serving as component A-I accounts for a proportion of the overall base resin which is from 100 to 30 wt %, preferably at least 50 wt %, more preferably at least 60 wt %, and even more preferably at least 70 wt %, but preferably not more than 92 wt %, more preferably not more than 89 wt %, and even more preferably not more than 86 wt %.

The metal salts of the copolymers of components A-I and A-II may be obtained by neutralizing some of the acid groups in the random copolymers of above components A-I and A-II with metal ions.

Examples of the metal ions which neutralize the acid groups include Na⁺, K⁺, Li⁺, Zn⁺⁺, Cu⁺⁺, Mg⁺⁺, Ca⁺⁺, Co⁺⁺, Ni⁺⁺ and Pb⁺⁺. Of these, Na⁺, Li⁺, Zn⁺⁺, Mg⁺⁺ and Ca⁺⁺ are preferred, and Zn⁺⁺ and Mg⁺⁺ are especially preferred.

In cases where a metal neutralization product is used in components A-I and A-II, i.e., in cases where an ionomer is used, the type of metal neutralization product and the degree of neutralization are not subject to any particular limitation. Illustrative examples include 60 mol % zinc (degree of neutralization with zinc) ethylene-acrylic acid copolymers, 40 mol % magnesium (degree of neutralization with magnesium) ethylene-acrylic acid copolymers, 40 mol % magnesium (degree of neutralization with magnesium) ethylene-methacrylic acid-isobutylene acrylate terpolymers, and 60 mol % Zn (degree of neutralization with zinc) ethylene-methacrylic acid-isobutylene acrylate terpolymers.

Illustrative examples of the olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymer serving as component A-I include those available under the trade names Nucrel AN4318, Nucrel AN4319, Nucrel AN4311, Nucrel N035C and Nucrel N0200H (DuPont-Mitsui Polychemicals Co., Ltd.). Illustrative examples of the metal salts of olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymers include those available under the trade names Himilan AM7316, Himilan AM7331, Himilan 1855 and Himilan 1856 (DuPont-Mitsui Polychemicals Co., Ltd.), and those available under the trade names Surlyn 6320 and Surlyn 8120 (E.I. DuPont de Nemours and Co., Ltd.).

The olefin-unsaturated carboxylic acid-unsaturated carboxylic acid random copolymer and/or metal salt thereof serving as component A-II has a weight-average molecular weight (Mw) of preferably at least 150,000, more preferably at least 160,000, and even more preferably at least 170,000, but preferably not more than 200,000, more preferably not more than 190,000, and even more preferably not more than 180,000. The weight-average molecular weight (Mw) to number-average molecular weight (Mn) ratio is preferably at least 3, and more preferably at least 4, but preferably not more than 7.0, and more preferably not more than 6.5.

The copolymer of component A-II accounts for a proportion of the overall base resin which is preferably from 0 to 70 wt %, more preferably at least 8 wt %, even more preferably at least 11 wt %, and most preferably at least 14 wt %, but preferably not more than 50 wt %, more preferably not more than 40 wt %, and even more preferably not more than 30 wt %.

Illustrative examples of the olefin-unsaturated carboxylic acid random copolymer serving as component A-II include those available under the trade names Nucrel 1560, Nucrel 1525 and Nucrel 1035 (DuPont-Mitsui Polychemicals Co., Ltd.). Illustrative examples of the metal salts of olefin-unsaturated carboxylic acid random copolymers include those available under the trade names Himilan 1605, Himilan 1601, Himilan 1557, Himilan 1705 and Himilan 1706 (DuPont-Mitsui Polychemicals Co., Ltd.); those available under the trade names Surlyn 7930 and Surlyn 7920 (E.I. DuPont de Nemours and Co., Ltd.); and those available under the trade names Escor 5100 and Escor 5200 (ExxonMobil Chemical).

In addition, to achieve a good rebound, use may be made of a highly neutralized ionomer in which the degree of neutralization has been enhanced by mixing components B and C below with above components A-I and A-II under applied heat.

In the present invention, within the above-described highly neutralized ionomeric resin composition, the following may be admixed per 100 parts by weight of the foregoing base resin composed primarily of components A-I and A-II:

-   (B) from 5 to 170 parts by weight of a fatty acid or fatty acid     derivative having a molecular weight of from 280 to 1500, and -   (C) from 0.1 to 10 parts by weight of a basic inorganic metal     compound capable of neutralizing acid groups within above components     A and B.

Component B is a fatty acid or fatty acid derivative having a molecular weight of at least 280 but not more than 1500 whose purpose is to enhance the flow properties of the heated mixture. It has a molecular weight which is much smaller than that of component A, and helps to significantly decrease the melt viscosity of the mixture. Also, because the fatty acid (or fatty acid derivative) of component B has a molecular weight of at least 280 but not more than 1500 and has a high content of acid groups (or derivative moieties thereof), its addition results in little if any loss of resilience.

The fatty acid or fatty acid derivative serving as component B may be an unsaturated fatty acid or fatty acid derivative having a double bond or triple bond in the alkyl moiety, or it may be a saturated fatty acid or fatty acid derivative in which all the bonds in the alkyl moiety are single bonds. It is recommended that the number of carbon atoms on the molecule be preferably at least 18, but preferably not more than 80, and more preferably not more than 40. Too few carbons may result in a poor heat resistance, and may also set the acid group content so high as to cause the acid groups to interact with acid groups present on the base resin, as a result of which the desired flow properties may not be achieved. On the other hand, too many carbons increases the molecular weight, which may lower the flow properties. In either case, the material may become difficult to use.

Specific examples of fatty acids that may be used as component B include stearic acid, 12-hydroxystearic acid, behenic acid, oleic acid, linoleic acid, linolenic acid, arachidic acid and lignoceric acid. Of these, preferred use may be made of stearic acid, arachidic acid, behenic acid, lignoceric acid and oleic acid.

The fatty acid derivative of component B is exemplified by derivatives in which the proton on the acid group of the fatty acid has been substituted. Exemplary fatty acid derivatives of this type include metallic soaps in which the proton has been substituted with a metal ion. Metal ions that may be used in such metallic soaps include Li⁺, Ca⁺⁺, Mg⁺⁺, Zn⁺⁺, Mn⁺⁺, Al⁺⁺⁺, Ni⁺⁺, Fe⁺⁺, Fe⁺⁺⁺, Cu⁺⁺, Sn⁺⁺, Pb⁺⁺ and Co⁺⁺. Of these, Ca⁺⁺, Mg⁺⁺ and Zn⁺⁺ are especially preferred.

Specific examples of fatty acid derivatives that may be used as component B include magnesium stearate, calcium stearate, zinc stearate, magnesium 12-hydroxystearate, calcium 12-hydroxystearate, zinc 12-hydroxystearate, magnesium arachidate, calcium arachidate, zinc arachidate, magnesium behenate, calcium behenate, zinc behenate, magnesium lignocerate, calcium lignocerate and zinc lignocerate. Of these, magnesium stearate, calcium stearate, zinc stearate, magnesium arachidate, calcium arachidate, zinc arachidate, magnesium behenate, calcium behenate, zinc behenate, magnesium lignocerate, calcium lignocerate and zinc lignocerate are preferred.

The amount of component B per 100 parts by weight of the base resin in the second member (II) is at least 5 parts by weight, preferably at least 20 parts by weight, more preferably at least 50 parts by weight, and even more preferably at least 80 parts by weight, but not more than 170 parts by weight, preferably not more than 150 parts by weight, even more preferably not more than 130 parts by weight, and most preferably not more than 110 parts by weight.

Use may also be made of known metallic soap-modified ionomers (see, for example, U.S. Pat. No. 5,312,857, U.S. Pat. No. 5,306,760 and International Disclosure WO 98/46671) when using above component A.

Component C is a basic inorganic metal compound capable of neutralizing the acid groups in above components A and B. As mentioned in prior-art examples, when components A and B alone, and in particular a metal-modified ionomeric resin alone (e.g., a metal soap-modified ionomeric resin of the type mentioned in the foregoing patent publications, alone), are heated and mixed, as shown below, the metallic soap and unneutralized acid groups present on the ionomer undergo exchange reactions, generating a fatty acid. Because the fatty acid has a low thermal stability and readily vaporizes during molding, it causes molding defects. Moreover, if the fatty acid thus generated deposits on the surface of the molded material, it will cause a substantial decline in paint film adhesion. Component C is included in order to resolve such problems.

The heated mixture of the second member (II) thus includes, as component C, a basic inorganic metal compound which neutralizes the acid groups present in above components A and B. The addition of component C confers excellent properties. That is, the acid groups in above components A and B are neutralized, and synergistic effects from the inclusion of each of these components increase the thermal stability of the heated mixture while at the same time imparting a good moldability and enhancing the rebound of the golf ball.

It is recommended that component C of the second member (II) be a basic inorganic metal compound—preferably a monoxide or hydroxide—which is capable of neutralizing acid groups in above components A and B. Because such compounds have a high reactivity with the ionomeric resin and the reaction by-products contain no organic matter, the degree of neutralization of the heated mixture can be increased without a loss of thermal stability.

The metal ions used here in the basic inorganic metal compound are exemplified by Li⁺, Na⁺, K⁺, Ca⁺⁺, Mg⁺⁺, Zn⁺⁺, Al⁺⁺⁺, Ni⁺, Fe⁺⁺, Fe⁺⁺⁺, Cu⁺⁺, Mn⁺⁺, Sn⁺⁺, Pb⁺⁺ and Co⁺⁺. Illustrative examples of the inorganic metal compound include basic inorganic fillers containing these metal ions, such as magnesium oxide, magnesium hydroxide, magnesium carbonate, zinc oxide, sodium hydroxide, sodium carbonate, calcium oxide, calcium hydroxide, lithium hydroxide and lithium carbonate. As noted above, a monoxide or hydroxide is preferred. The use of magnesium oxide or calcium hydroxide, which have high reactivities with ionomeric resins, is especially preferred.

The above basic inorganic metal compound serving as component C is an ingredient for neutralizing the acid groups in above components A and B and is included in an amount, based on the acid groups in above components A and B, of preferably at least 30 mol %, more preferably at least 45 mol %, even more preferably at least 60 mol %, and most preferably at least 70 mol %, but preferably not more than 130 mol %, more preferably not more than 110 mol %, even more preferably not more than 100 mol %, still more preferably not more than 90 mol %, and most preferably not more than 85 mol %. In this case, the amount in which the basic inorganic metal compound serving as component C is included may be suitably selected so as to achieve the desired degree of neutralization. The component C in the invention is included in an amount, expressed on a weight basis per 100 parts by weight of the base resin, of preferably from 0.1 to 10 parts by weight, more preferably at least 0.5 part by weight, even more preferably at least 0.8 part by weight, and most preferably at least 1 part by weight, but preferably not more than 8 parts by weight, more preferably not more than 5 parts by weight, and even more preferably not more than 4 parts by weight.

The above resin composition has a melt flow rate, as measured in accordance with JIS-K6760 (test temperature, 190° C.; test load, 21 N (2.16 kgf)), of preferably at least 1 g/10 min, more preferably at least 2 g/10 min, and even more preferably at least 3 g/10 min, but preferably not more than 30 g/10 min, more preferably not more than 20 g/10 min, even more preferably not more than 15 g/10 min, and most preferably not more than 10 g/min. If the melt index of this resin mixture is low, the processability of the mixture may markedly decrease.

The method of preparing the above resin mixture is not subject to any particular limitation, although use may be made of a method which involves charging the ionomers or unneutralized polymers of components A-I and A-II, together with component B and component C, into a hopper and extruding under the desired conditions. Alternatively, component B may be charged from a separate feeder. In this case, the neutralization reaction by above component C as the metal cation source with the carboxylic acids in components A-I, A-II and B may be carried out by various types of extruders. The extruder may be either a single-screw extruder or a twin-screw extruder, although a twin-screw extruder is preferable. Alternatively, these extruders may be used in a tandem arrangement, such as single-screw extruder/twin-screw extruder or twin-screw/twin-screw extruder. These extruders need not be of a special design; the use of existing extruders will suffice.

The golf ball of the invention includes the above-described hot-molded material (first member (I)) and the above-described resin composition (second member (II)), but the manner in which these members are included in the golf ball is not subject to any particular limitation. Illustrative examples of suitable embodiments of the golf ball include (1) to (4) below.

-   (1) A two-piece solid golf ball in which the first member (I) serves     as the core, and the second member (II) serves as the cover. -   (2) A multi-piece solid golf ball in which the first member (I)     serves as the core, and the second member (II) serves as an inner     cover layer and is encased with one or more outer cover layer. -   (3) A multi-piece solid golf ball in which the first member (I)     serves as the core, the second member (II) serves as an outer cover     layer, and one or more inner layer cover is interposed therebetween. -   (4) A multi-piece solid golf ball in which the second member (II)     serves as the core, and the first member (I) serves as an inner     cover layer and is encased by one or more outer cover layer.

Known outer cover layer materials and inner cover layer materials may be used as the outer cover layer material and inner cover layer material in the respective above embodiments. These outer cover layer materials and inner cover layer materials are exemplified by thermoplastic or thermoset polyurethane elastomers, polyester elastomers, ionomeric resins, polyolefin elastomers, or mixtures thereof. The use of thermoplastic polyurethane elastomers and ionomeric resins is especially preferred. These may be used singly or as combinations of two or more thereof.

Of the above-described Embodiments (1) to (4), the use of Embodiment (2) is especially preferred. In this case, although the material of the one or more outer cover layer which encases the core and the inner cover layer is not subject to any particular limitation, the outer cover layer may be produced using a known material. By way of illustration, the outer cover layer material may be composed primarily of a thermoplastic or thermoset polyurethane elastomer, a polyester elastomer, an ionomeric resin, an ionomeric resin having a relatively high degree of neutralization, a polyolefin elastomer or a mixture thereof. These may be used singly or as mixtures of two or more thereof. The use of a thermoplastic polyurethane elastomer, an ionomeric resin, or an ionomeric resin having a relatively high degree of neutralization is especially preferred.

Illustrative examples of thermoplastic polyurethane elastomers that may be used for the above purpose include commercial products in which the diisocyanate is an aliphatic or aromatic compound, such as Pandex T7298, Pandex T7295, Pandex T7890, Pandex TR3080, Pandex T8290, Pandex T8295 and Pandex T1188 (all manufactured by DIC Bayer Polymer, Ltd.). Illustrative examples of commercial ionomeric resins include Surlyn 6320, Surlyn 8945, Surlyn 9945, and Surlyn 8120 (all products of E.I. DuPont de Nemours and Co., Inc.), and Himilan 1706, Himilan 1605, Himilan 1855, Himilan 1557, Himilan 1601 and Himilan AM7316 (all products of DuPont-Mitsui Polychemicals Co., Ltd.).

Together with the primary material described above, the outer cover layer material may include also, as an optional ingredient, a polymer other than the foregoing thermoplastic elastomers. Specific examples of polymers that may be included as optional ingredients include polyamide elastomers, styrene block elastomers, hydrogenated polybutadienes and ethylene-vinyl acetate (EVA) copolymers.

The golf ball of the invention can be manufactured by any suitable known method without particular limitation. In one preferred method, the solid core is placed within a given injection mold, following which a given technique is used to inject over the core the above-described inner cover layer material and outer cover layer material in this order. In another preferred method, these cover layer materials are formed into pairs of half-cups, and the resulting pairs of half-cups are successively placed over the solid core and compression-molded.

In the golf ball of the invention, it is preferable for the outer cover layer in the above Embodiment (2) to have a lower Shore D hardness than the inner cover layer. However, the invention is not limited to this hardness relationship.

The inner cover layer has a Shore D hardness of preferably at least 50, more preferably at least 51, even more preferably at least 52, and most preferably at least 53, but preferably not more than 80, more preferably not more than 75, even more preferably not more than 70, and most preferably not more than 65.

It is recommended that the outer cover layer have a Shore D hardness of at least 35, preferably at least 40, more preferably at least 45, and most preferably at least 48, but not more than 60, preferably not more than 58, more preferably not more than 56, and most preferably not more than 54.

In above Embodiment (2), it is recommended that the outer cover layer have a lower Shore D hardness than the inner cover layer, and that the Shore D hardness difference between the outer cover layer and the inner cover layer be preferably at least 2, more preferably at least 5, even more preferably at least 7, and most preferably at least 9, but preferably not more than 30, more preferably not more than 25, and even more preferably not more than 20.

In above Embodiment (2), it is recommended that the inner cover layer and the outer cover layer have respective thicknesses of preferably at least 0.1 mm, more preferably at least 0.5 mm, and even more preferably at least 0.7 mm, but preferably not more than 5.0 mm, more preferably not more than 3.0 mm, even more preferably not more than 2.0 mm, and most preferably not more than 1.8 mm.

The golf ball of the invention may be manufactured for competitive use in accordance with the Rules of Golf; that is, to a diameter of not less than 42.67 mm and a weight of not more than 45.93 g. It is recommended that the diameter be preferably not more than 44.0 mm, more preferably not more than 43.5 mm, and most preferably not more than 43.0 mm; and that the weight be preferably not less than 44.5 g, more preferably not less than 45.0 g, even more preferably not less than 45.1 g, and most preferably not less than 45.2 g.

The golf ball of the invention has an excellent resilience for the ball as a whole, a good, soft feel on impact, and imparts an excellent spin performance, thereby enabling the distance to be increased. In addition, it has a high manufacturability.

EXAMPLES

The following Examples and Comparative Examples are provided by way of illustration, and not by way of limitation.

Examples 1 to 3 Comparative Examples 1 to 6

Cores (spherical moldings) having a diameter of 36.4 mm and a weight of 29 g were produced by working together, with a kneader, the respective starting materials in the proportions shown in Table 1 below so as to prepare a rubber composition, then carrying out 20 minutes of vulcanization at 160° C. in a spherical mold.

TABLE 1 Core No. No. 1 No. 2 No. 3 No. 4 Formulation Polybutadiene EC140 80 20 20 100 (pbw) Polybutadiene BR51 20 80 Polybutadiene BR11 80 Organic peroxide 1.4 1.4 1.4 1.4 Zinc oxide 18 18 18 18 Antioxidant 0.1 0.1 0.1 0.1 Zinc acrylate 27 27 27 27 Zinc salt of pentachloro- 1 1 1 1 thiophenol Extrusion workability Exc Exc Exc NG

Details of the above formulation are provided below.

-   Polybutadiene rubber: EC140 (trade name), available from Firestone     Polymers. Polymerized with a neodymium catalyst. Mooney viscosity,     43; T₈₀ value, 2.3. -   Polybutadiene rubber: BR51 (trade name), available from JSR     Corporation. Polymerized with a neodymium catalyst. Mooney     viscosity, 39; T₈₀ value, 5.0. -   Polybutadiene rubber: BR11 (trade name), available from JSR     Corporation. Polymerized with a nickel catalyst. Mooney viscosity,     44; T₈₀ value, 4.9. -   Organic peroxide: Dicumyl peroxide, available from NOF Corporation     under the trade name Percumyl D. -   Zinc oxide: Available from Sakai Chemical Industry Co., Ltd. under     the trade name Sanshu Sanka Aen. Average particle size, 0.6 μm (air     permeametry). -   Antioxidant: Available from Ouchi Shinko Chemical Industry Co., Ltd.     under the trade name Nocrac NS-30. -   Zinc acrylate: Available from Nippon Shokubai Co., Ltd. under the     trade name ZN-DA85S.     Extrusion Workability

In each example, the rubber composition was extruded under the same extrusion conditions and prepared into slabs.

The extrusion workability was rated according to the following criteria.

-   -   Exc: The rubber slab had a smooth surface texture and a stable         weight.     -   Good: The rubber slab had a rough surface texture and an         unstable weight.     -   NG: A slab having a fixed shape was difficult to obtain.

The deflection of the resulting core when compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf) was determined. The results are given in Table 3.

Next, the core obtained was placed in a given mold, and the resin shown in Table 2 (A, B, C, X, Y or Z) was injection-molded, thereby forming an inner cover layer-encased core having a diameter of about 39.7 mm. The inner cover layer-encased core was then transferred to another mold and the resin shown in Table 2 (D, E or F) was injection-molded so as to produce a three-piece solid golf ball having a diameter of about 42.7 mm and a weight of about 45.3 g. Trade names of the chief ingredients used are indicated below.

-   Himilan: Ionomeric resins produced by DuPont-Mitsui Polychemicals     Co., Ltd. -   Surlyn: Ionomeric resins produced by E.I. DuPont de Nemours & Co. -   Dynaron: A butadiene-styrene block copolymer hydrogenation product     produced by JSR Corporation. -   Pandex: Thermoplastic polyurethane elastomers produced by DIC-Bayer     Polymer.

TABLE 2 Formulation (pbw) A B C D E F X Y Z Himilan 1706 50 Himilan 1605 50 Himilan 1557 20 Himilan 1855 30 Surlyn 8945 35 Surlyn 9945 35 Surlyn 8120 100 50 Dynaron 6100P 30 Pandex T8290 50 Pandex T8295 50 100 Titanium dioxide 4 4 4 2.7 2.7 4 4 4 4 Nucrel AN4319 75 40 Nucrel N035C 40 Surlyn 6320 60 Nucrel N1560 25 Escor 5100 60 Oleic acid 25 Magnesium stearate 69 100 Magnesium oxide 3.6 0.8 2.8

The performances of the golf balls obtained were examined as follows. The results are shown in Table 3.

Material Properties

The Shore D hardnesses of the inner cover layer and the outer cover layer are shown as the surface hardnesses of the respective materials, as measured with a durometer by the test method described in ASTM D2240.

Golf Ball Properties

The carry and total distance were measured when the ball was hit at a head speed of 50 m/s with a driver (W#1) mounted on a swing machine.

Feel

The feel of the ball when actually shot with a driver (number one wood) and a putter was rated by five professional golfers and five top-caliber amateur golfers as “too hard,” “good” or “too soft.” The rating assigned most often to a particular ball was used as that ball's overall rating.

Spin on Approach Shots

The spin rate of the golf ball when hit at a head speed (HS) of 20 m/s using a sand wedge (SW) mounted on a swing robot was measured.

TABLE 3 Example Comparative Example 1 2 3 1 2 3 4 5 6 7 Core Type 1 1 1 1 1 1 2 3 2 4 Diameter 36.4 36.4 36.4 36.4 36.4 36.4 36.4 36.4 36.4 36.4 (mm) Weight (g) 29 29 29 29 29 29 29 29 29 29 Deflection 3.7 3.7 3.7 3.7 3.7 3.7 3.7 3.6 3.7 3.7 (mm) Inner Type X Y Z A B C B B X B cover Shore D 53 56 54 63 56 45 56 56 53 56 layer hardness Specific 0.97 0.97 0.97 0.98 0.97 0.98 0.97 0.97 0.97 0.97 gravity Gauge (mm) 1.65 1.65 1.65 1.65 1.65 1.65 1.65 1.65 1.65 1.65 Outer Type E E E D E F E E E E cover Shore D 51 51 51 47 51 53 51 51 51 51 layer hardness Specific 1.18 1.18 1.18 1.18 1.18 0.98 1.18 1.18 1.18 1.18 gravity Gauge (mm) 1.50 1.50 1.50 1.49 1.50 1.50 1.50 1.50 1.50 1.50 Ball Carry (m) 240.5 241.0 241.3 238.2 237.7 228.3 226.2 224.0 228.5 237.7 properties Total 271.4 271.9 272.3 268.4 268.3 258.0 252.7 250.1 258.3 268.4 (#W1, distance (m) HS 50) Spin rate 3074 3105 3085 3305 3340 3400 3319 3335 3085 3332 (rpm) Feel good good good good good too good good good good soft Spin rate on approach 6415 6522 6474 6711 6605 6561 6268 6203 6420 6610 shot (rpm) (Sand wedge, HS 20) Feel on shots with good good good good good too good good good good putter soft

As is apparent from the results shown in Table 3, compared with the golf balls in the examples of the invention, the golf balls in Comparative Examples 1 to 7 failed to travel a sufficient distance, had a poor feel on impact, and had a poor rubber extrusion workability. 

The invention claimed is:
 1. A golf ball comprising a plurality of members which include: (I) a first member comprised of a material molded under heat from a rubber composition comprising (a) a base rubber composed primarily of a mixture of at least 60 wt % of a polybutadiene having a stress relaxation time (T₈₀), defined as the length of time it takes from the moment rotor rotation is stopped immediately after measurement of the ML₁₊₄ (100° C.) value (the Mooney viscosity measured at 100° C. in accordance with ASTM D-1646-96) for the ML₁₊₄ value to decrease 80%, of less than 4 seconds and up to 40 wt % of a polybutadiene having a stress relaxation time (T₈₀) of at least 4 seconds, (b) an unsaturated carboxylic acid and/or a metal salt thereof, and (c) an organic peroxide; and (II) a second member formed primarily of a resin composition obtained by mixing 100 parts by weight of a base resin composed primarily of (A-I) an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymer and/or a metal salt thereof, and/or (A-II) an olefin-unsaturated carboxylic acid random copolymer and/or a metal salt thereof, (B) from 100 to 170 parts by weight of a fatty acid or fatty acid ester having a molecular weight of from 280 to 1500, and (C) from 0.1 to 10 parts by weight of a basic inorganic metal compound capable of neutralizing acid groups in components A and B, wherein (i) the olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymer and/or a metal salt thereof serving as component A-I has a weight-average molecular weight (Mw) of at least 100,000 but not more than 200,000, and a weight-average molecular weight (Mw) to number-average molecular weight (Mn) ratio of at least 3 but not more than 7.0, and (ii) the olefin-unsaturated carboxylic acid random copolymer and/or a metal salt thereof serving as component A-II has a weight-average molecular weight (Mw) of at least 150,000 but not more than 200,000, and a weight-average molecular weight (Mw) to number-average molecular weight (Mn) ratio of at least 3 but not more than 7.0.
 2. The golf ball of claim 1, wherein the rubber composition further comprises (d) an organosulfur compound.
 3. The golf ball of claim 1, wherein the polybutadiene having a stress relaxation time (T₈₀) of less than 4 seconds and the polybutadiene having a stress relaxation time (T₈₀) of at least 4 seconds are both polybutadienes prepared using a rare-earth catalyst.
 4. The golf ball of claim 1, wherein at least the polybutadiene having a stress relaxation time (T₈₀) of less than 4 seconds or the polybutadiene having a stress relaxation time (T₈₀) of at least 4 seconds is a polybutadiene prepared using a rare-earth catalyst.
 5. The golf ball of claim 4, wherein at least one of the polybutadienes is a polybutadiene prepared by polymerization using a rare-earth catalyst, followed by terminal modification.
 6. The golf ball of claim 1, wherein the resin composition has a melt flow rate, as measured in accordance with JIS-K6760 (test temperature, 190° C.; test load, 21 N (2.16 kgf)), of at least 1 g/10 min but not more than 30 g/10 min.
 7. The golf ball of claim 1, wherein the terpolymer of component A-I accounts for a proportion of the overall base resin of from 100 to 30 wt %, and the copolymer of component A-II accounts for a proportion of the overall base resin of from 0 to 70 wt %.
 8. The golf ball of claim 1, wherein the base resin in the second member (II) is composed primarily of the olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymer and/or a metal salt thereof serving as component A-I.
 9. The golf ball of claim 1 which is a golf ball having a core, an inner cover layer and an outer cover layer, wherein the first member (I) is used as the core and the second member (II) is used as the inner cover layer.
 10. The golf ball of claim 9, wherein the inner cover layer has a Shore D hardness of from 50 to 80, and the outer cover layer has a Shore D hardness which is from 35 to 60 and is lower than the Shore D hardness of the inner cover layer. 